fix: use pull_request.head.sha for fork PR checkout

github.head_ref is empty for fork PRs, causing checkout to fail.
Use github.event.pull_request.head.sha as primary ref with
github.head_ref as fallback for branch pushes.

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.sha || github.head_ref }}
 
       - name: Install nightly toolchain for rustfmt
         run: rustup install nightly --profile minimal --component rustfmt
@@ -36,7 +36,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.sha || github.head_ref }}
 
       - name: Install taplo-cli
         run: cargo install --locked taplo-cli
@@ -49,7 +49,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.sha || github.head_ref }}
 
       - name: Install active toolchain
         run: rustup install
@@ -65,7 +65,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.sha || github.head_ref }}
 
       - name: Install cargo-deny
         run: cargo install --locked cargo-deny
@@ -81,7 +81,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.sha || github.head_ref }}
 
       - uses: ./.github/actions/install-system-deps
 
@@ -110,7 +110,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.sha || github.head_ref }}
 
       - uses: ./.github/actions/install-system-deps
 
@@ -138,7 +138,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.sha || github.head_ref }}
 
       - uses: ./.github/actions/install-system-deps
 
@@ -168,7 +168,7 @@ jobs:
   #   steps:
   #     - uses: actions/checkout@v5
   #       with:
-  #         ref: ${{ github.head_ref }}
+  #         ref: ${{ github.event.pull_request.head.sha || github.head_ref }}
 
   #     - uses: ./.github/actions/install-system-deps
 
@@ -196,7 +196,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.sha || github.head_ref }}
 
       - uses: ./.github/actions/install-system-deps
 
@@ -222,7 +222,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.sha || github.head_ref }}
 
       - uses: ./.github/actions/install-risc0